1. Field of the Invention
The present invention relates to a surface modification technique for a carbon nanotube material.
2. Description of the Related Art
In recent years, carbon nanotubes (CNTs) are being studied for electronic components with the properties of electrical conductors and thermal conductors in the field of semiconductor devices and semiconductor integrated circuit devices including printed wiring boards.
There is particular interest in CNTs as materials for forming semiconductor devices because of their excellent chemical stability, unique physical and electrical properties and other characteristics, and various researches are continuing into means of controlling their thickness and length, formation position, chirality and the like.
Specific applications of interest include electromagnetic shielding materials in electronics, cooling bump component materials for ultra-LSI and other advanced electronic devices, and structural components for the wiring vias of semiconductor devices (electronic devices) and the like.
For example, one application that is being considered to exploit the extremely good thermal conductivity of CNTs is to grow them at high densities on semiconductor process boards or to scatter previously-synthesized CNTs on boards to thereby form and mount CNT parts which can be used as adhesive structures for semiconductor devices (electronic devices) mounted on some electrically conductive circuits and process boards, and exhaust heat paths (so called “bump structures”) for the device heat discharged by the adhesive structures.
Furthermore, the extremely good electrical conductivity of CNTs could make them useful for application to the via wiring structural body of the high-density wiring structures of semiconductor devices having ultrafine structures.
FIG. 5 shows one example of a structure (see for example the technical bulletin “Fujitsu Pioneers Use of Carbon Nanotubes for Heatsinks for Semiconductors”, by Fujitsu Ltd., Fujitsu Laboratories Ltd. 5 Dec. 2005 that uses such CNTs as the cooling bump material of an advanced electronic device. As shown in FIG. 5, regarding the cooling bump structure for such an advanced electronic device, the CNT bump structure can be prepared for example by depositing a catalytic metal carrier film (such as a TiN film) and a catalytic metal film (Co or the like) (shown together as 53 in the drawing) by sputtering or the like on electrode 52 on substrate (aluminum nitride (AlN), alumina or the like) 51, then growing CNTs 54 by thermal CVD (chemical vapor deposition) using hydrocarbon gas (CH4, C2H2 or the like), and then applying a conductive substance (Cu, Al or other metal or the like) to the CNT part of the board with CNTs by plating (wet processing) or the like. An electronic device can then be thermo-compression bonded (preferably at about 250 to 450° C.) on this board to prepare a highly thermally conductive electronic device.
FIG. 1 shows one example of a wiring via structure using the aforementioned CNTs (see for example Japanese Unexamined Patent Application Publication No. 2002-329723 (Claims) and Nihei et al., Japanese Journal of Applied Physics, 2005, Vol. 44, p. 1626). As shown in FIG. 1, such a via structure can be prepared for example by forming underlayer 2 and Cu wiring layer 3 on substrate 1, depositing barrier film (Ta film or the like) 4 on Cu wiring layer 3 to prevent scattering of the Cu, forming insulating layer 5 on top of that, making the via hole, and then depositing catalytic metal carrier film (Ti or the like) 6 and Co or other catalytic metal film (or catalytic fine particle layer) 7 by sputtering or the like, growing CNTs 8 by thermal CVD (chemical vapor deposition) or the like using hydrocarbon gas (CH4, C2H2 or the like), and finally forming the upper wiring. FIG. 1 also shows filler resin 9 for fixing CNTs 8.
However, the problem is that although the CNTs themselves have excellent electrical conductivity, semiconductor properties, thermal conductivity, chemical stability and the like, they may not have sufficient affinity to the materials with which they come into contact, so that electrical and thermal conductivity may be greatly reduced at the connections and adequate adhesiveness and close contact between layers may not be obtained. This problem also occurs when nanotubes manufactured by CVD with one end fixed to a Si substrate are used for wiring purposes.
One potential solution would be to achieve perfect close contact between the CNTs and surrounding layers when manufacturing the component. The problem is, however, that such contact cannot be achieved without resolving the problem of poor affinity at the boundaries to the other materials. This problem is common to all applications of CNTs.
CNTs are conventionally manufactured by various manufacturing methods including laser ablation, chemical vapor deposition (CVD), the HiPCo (high-pressure carbon monoxide) method and the like. The surface properties of CNTs manufactured by these methods are dependent on the properties of the graphite-like surface molecular structure, which is an electronic hyperconjugated molecular structure of linked benzene rings, and are also graphite-like in terms of wettability with other materials. That is, the molecular surface after manufacture (as a powder for example) without any additional treatment normally has poor dispensability in any solvents, and even treatment under specific conditions (such as ultrasonic treatment in the presence of ethanol) can only provide a dispersed state for a few weeks at most.
As discussed before, this property has greatly restricted various engineering applications of CNTs. That is, when attempting to manufacture hybrid materials of manufactured CNTs with other materials, such as functional structural materials mixed with resins, it is currently difficult to manufacture a composite material with sufficient compatibility on the microscopic level by kneading or otherwise processing the CNTs with other materials without surfactants and other additives, and when additives are added, the properties of these materials inevitably have adverse effects on the composite material, resulting in, for example, poorer electrical properties, mechanical strength and chemical properties. Poorer electrical properties here mean, for example, increased specific resistance, decreased reliability of the electrical properties in the middle to long term, increased specific resistance relative to weight and poorer electromagnetic shielding performance, as well as poorer reliability in the performance. Poorer mechanical strength means decreased rigidity and breaking strength, as well as long-term deterioration of these properties and the like. Poorer chemical properties mean that the material properties involving the environment (such as hygroscopicity, solvent resistant, oxidation from oxygen in the air) deteriorate over time.
For example, in order to use CNTs as via wiring materials in ultra-LSI and other high-density advanced electronic devices, the tops of CNTs grown in a via must be shaved by CMP (chemical mechanical polishing). In this case, it may be necessary to fill or fix the area around the bundle of CNTs with an insulating material or the like in order to fix the bundle or prevent polishing material or liquid from infiltrating and contaminating the CNTs during CMP (or to make it easier to remove when it does infiltrate), but if the CNTs do not have a good affinity to the insulating material or the like, the insulating material may not completely fill the spaces between CNT bundles even if a resin dissolved in a solvent is applied by spin coating or the like, or even if a film is formed from the resin material in a vacuum environment.